Detonating cord and methods of making and using the same

ABSTRACT

The velocity of detonation of an explosive such as detonating cord ( 18, 22 ) is controlled by the addition of a diluent to the explosive, e.g., to the core of the detonating cord ( 18, 22 ). An explosively inert diluent, or a diluent comprised of an explosive of lower brisance than the principal explosive comprising the core of the detonating cord, will serve to reduce the velocity of detonation. Such reduced velocity of detonation has beneficial effects in certain operations, including cleaving rock ( 10 ), wherein it is observed to significantly reduce radial cracks ( 24 ) and stickers ( 26 ) (long radial cracks) in the vicinity of the boreholes ( 12 ) in which the low-velocity detonating cord ( 18, 22 ) is functioned to cleave the rock ( 10 ). The low-velocity detonating cord also facilitates leaving behind a smoother face in cutting trenches and tunnels through rock. The method of manufacture of low-velocity detonating cord includes incorporating a suitable diluent, such as phenolic microballoons, into an explosive core of, e.g., PETN.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of patent application Ser. No.11/622,252, filed Jan. 11, 2007, entitled “Detonating Cord and Methodsof Making and Using the Same”, which is a continuation of patentapplication Ser. No. 09/863,795, filed May 23, 2001, entitled“Detonating Cord and Methods of Making and Using the Same”, which claimsthe benefit of U.S. provisional patent application Ser. No. 60/206,877,filed May 24, 2000, entitled “Detonating Cord Having Controlled Velocityof Detonation and Methods of Making and Using the Same”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is concerned with detonating cord having acontrolled energy release which is attained by incorporating a diluentinto the explosive core of the cord to control the velocity ofdetonation of the core. The present invention is also concerned with amethod of making the detonating cord, and a method of utilizing thedetonating cord to effectuate desired cutting or rupturing of anystructures, rock formations or the like while minimizing undesiredancillary damage.

2. Related Art

Detonating cord is, of course, well known in the art and comprises asolid core of high explosive encased in a protective jacket which isusually waterproofed, such as by being coated with a suitable syntheticpolymeric (plastic) material. Typically, the solid core of highexplosive is a compressed pulverulent explosive which may or may not beplastic-bonded. Detonating cords are made in various sizes (corediameters) conventionally measured in grains of explosive per unitlength. A typical explosive core for detonating cord is pentaerythritoltetranitrate (“PETN”) and typical core sizes range from about 5 grainsof explosive per linear foot of cord (“gr/ft”) to about 400 gr/ft. Thereare 15.432 grains per gram, so that, e.g., 100 gr/ft is, in the metricsystem, about 21.3 grams per meter (“g/m”). Typical velocities ofdetonation for detonating cord made of PETN are on the order of about6,500 to 7,500 meters per second (“m/sec”).

In order to provide a detonating cord having explosive and otherproperties which are uniform along its length, it is necessary tocompress the explosive core in order to standardize the density of thecore along its length because the velocity of detonation, and therebythe explosive energy output, is proportional to the density of theexplosive core. Generally, increased density of the core increases thevelocity of detonation and thereby the explosive energy output per unitlength of the cord. As is well known, the particle size of the explosivealso greatly affects the velocity of detonation and a critical diameterexists for propagation of the explosion along the length of thedetonating cord. Generally, as the diameter of the explosive core of thedetonating cord decreases, so should the particle size. For example, fora small detonating cord, e.g., one containing about 5 gr/ft (1.1 g/m) ofexplosive, a particle size of about 20 microns diameter is suitable,whereas for a detonating cord of much larger diameter, e.g., adetonating cord of about 400 gr/ft (85.2 g/m), adequate propagation ofthe explosion along the length of the cord may be attained with aparticle diameter size of from 100 to 200 microns.

In commercial blasting operations, detonating cord is generally used totransfer an explosive signal to various components of a blasting setup.For example, detonating cord may be utilized as a surface trunkline toimpart a detonation signal to a series of down-hole fuses such as shocktube or other detonating cords. While, as noted above, PETN is the usualchoice of explosive for detonating cord, other explosives may be used.For example, for operations such as those in the oil and gas industry inwhich high temperatures are experienced by the detonating cord before itis to be functioned (initiated), explosives such ascyclo-1,3,5-trimethylene-2,4,6-trinitramine (Cyclonite, or “RDX”) orcyclotetramethylene tetranitramine (Homocyclonite, or “HMX”) may beutilized for the core of the detonating cord. The explosiveshexanitrostilbene (“HNS”), tetranitrocarbazol (“TNC”), and 2-6, bispicryoamino 3,5, dinitro pyridine (“PYX”) are among other explosiveswhich may be used as the first explosive material of the core. It isalso known, usually in military operations, to utilize detonating cordto sever structural members such as the beams or braces of a bridge,trees, etc. As to the tubular sheath which encloses the core ofexplosive material, any suitable material or combinations thereof, as iswell known in the art, may be employed. Such sheaths are pliable enoughto enable the detonating cord to be deployed in any desired pattern,wrapped around structural members, etc. The sheath may also be a rigidsheath such as that described in patent application Ser. No. 09/645,276,filed on Aug. 24, 2000 in the name of Mark E. Woodall et al for “RigidReactive Cord And Methods Of Use And Manufacture”. That patentapplication describes a detonating cord having a non-metal outer sheathwhich imparts a sufficient flexural modulus, e.g., of about 250,000 psi(17.236×10² MPa), which enables a 6-foot length of the cord to besufficiently rigid to perforate and penetrate fly ash. This rigid-typecord finds use in removing fly ash from boiler tubes.

It is known in the explosives art to utilize glass microballoons assensitizing agents in emulsion explosives and the like. In this regard,see U.S. Pat. No. 6,165,297 issued Dec. 26, 2000 to J. G. B. Smith etal, entitled “Process And Apparatus For The Manufacture Of EmulsionExplosive Compositions”. At column 1, lines 41-45, it is noted thatsensitizing agents such as glass microballoons may be a component ofemulsion explosives. Column 2, lines 6-7 and column 2, lines 63-67 ofthe same patent, describe microballoons as a species of “closed cellvoid material.” Similarly, U.S. Pat. No. 6,200,398, issued Mar. 13, 2001to J. H. Bush and entitled “Emulsion Explosive Compositions” discloses,at column 24, line 46 to column 25, line 35 the use of closed-cell voidcontaining materials such as discrete glass spheres having a particlesize within the range of about 10 to about 175 microns and a bulkdensity within the range of about 0.1 to about 0.4 g/cc. Various othermicroballoons are described. U.S. Pat. No. 5,714,711, issued Feb. 3,1998 to J. B. Schumacher et al, is entitled “Encapsulated PropellantGrain Composition, Method Of Preparation, Article Fabricated TherefromAnd Method Of Fabrication”. This patent deals with a propellant graincomposition for use in solidfuel rocket engines and discloses anoxidizer first reactant encapsulated by a polymeric barrier coating anda reducer fuel second reactant disposed on the polymeric barriercoating, with a final polymeric coating placed over the entirepropellant grain to yield a unitary metal fueloxidizer propellant grainstructure for use as a solid rocket fuel. U.S. Pat. No. 5,859,264 issuedJan. 12, 1999 to K. Coupland et al is entitled “Explosive Compositions”and discloses emulsifiers for use in emulsion explosives comprising acontinuous organic phase and a discontinuous aqueous phase. At column 2,lines 59-67, this patent discloses the use of glass or resinmicrospheres or other gas-containing particulate materials.

In known mining, construction and quarrying operations, explosives areused to break a web of rock extending along a line of relatively closelyspaced, small diameter parallel boreholes, in order to cleave the rockmass along the line of boreholes. Any cracking, spalling orfragmentation of stone not contributing to this cleaving is undesirable.In construction and mining operations, it is desired to leave behindrelatively smooth walls and roofs in cuts and tunnels, and in quarryingoperations the objective is to recover from the rock formation blocks ofstone which are as undamaged as possible. In some cases, the cleaving isperformed by using mechanical wedges instead of explosives. Wedges areplaced in each borehole of a line of boreholes and each wedge isgradually mechanically loaded in order to develop an even tensile stressfield along the web of stone extending between and connecting the row ofboreholes, until the web fragments to cleave the rock mass.

Blasting methods attempt to mimic the mechanical splitting process. Inmost cases, however, the shock energy from a high explosive will cause adiscrete stress field to form around individual boreholes beforesubsequent boreholes are loaded by the explosive force, and impose highparticle velocities to fractured rock particles, both factors resultingin undesired radial fracture damage around the boreholes. Statedotherwise, when the energy shock wave engendered by the explosive gasesgenerated from the explosive charges placed in the boreholes pressurizesthe holes and fragments the web of rock between the holes,high-amplitude shock energy exacerbates unwanted spalling and crackingalong the radial fractures emanating from the boreholes and associateddamage to the rock.

The dimension stone industry is concerned with cutting from rockformations in quarries stone which is sized for use in construction andfor headstones, markers and the like. In the dimension stone industry,black powder was one of the original explosive materials used inboreholes for cleaving stone by blasting. Black powder has a very lowvelocity of detonation and a very low explosive output and shock energy.These characteristics are advantageous in reducing collateral damage tostone cut from rock formations. The disadvantages of black powder aresafety problems which inhere in its use, because black powder isextremely sensitive to static, sparks and fire, making it extremelydangerous. Black powder is also de-activated by water, precluding itsuse in wet areas.

An alternative to black powder in cleaving stone by blasting isdynamite. Dynamite is nitroglycerine soaked into an absorbent materialand packaged in a cylindrical cartridge. The velocity of detonation ofdynamite, about 4500 feet per second (about 1,372 meters per second), isslightly higher than that of black powder. Dynamite also has slightlymore radial explosive output. The primary disadvantage of dynamite isthe excruciating headaches experienced by personnel who handlenitroglycerine-based material. Dynamite may also require a relativelylarge explosive diameter to function, rendering it unusable forsmaller-diameter boreholes.

SUMMARY OF THE INVENTION

Although it is generally desired to have a high explosive output for agiven diameter detonating cord, it has been found that for someapplications it is desirable to control, e.g., to reduce, the velocityof detonation because such reduction reduces the peak output shock wavepressure caused by the explosion. High peak outlet shock wave pressurecauses rapid pressure loading of the structure, e.g., rock, beingruptured by the detonating cord, which nucleates fractures causing themto spread into portions of the structure which it is desired to leaveintact. A reduction in peak shock wave output pressure, i.e., areduction in the peak amplitude of the shock energy released bydetonation of the cord, has been found to be highly beneficial in someapplications where high-amplitude shock energy may cause or exacerbateunwanted collateral damage. Such applications include certainconstruction and tunneling activities, quarrying operations and cuttingstructures such as dimension stone, as described more fully below.

Generally, in accordance with the present invention there is provided adetonating cord having a controlled velocity of detonation andcomprising a solid core of an explosive containing therein a firstexplosive and one or more diluents which reduce the velocity ofdetonation of the core. The detonating cord of the present inventionfinds use in any application in which reduced peak shock energy isrequired or desired.

Reference herein to “reduced-velocity detonating cord”, “low-velocitydetonating cord”, or the like, means a detonating cord whose explosivecore contains a diluent which reduces the velocity of detonation of thedetonating cord as compared to an otherwise identical detonating cordwhich does not contain the diluent.

The diluent may be either an explosively inert material, such asclosed-cell void materials (referred to herein as microballoons, e.g.,glass or resin microballoons or very fine plastic or glass beads, etc.,or it may be an explosive material, for example, ammonium nitrate. Asused herein, reference to a “solid” core of explosive material meansthat the tubular sheath of the detonating cord is completely filled withthe explosive material. The presence of microballoons dispersed in theexplosive material provides enclosed voids therein, but as the explosivematerial substantially completely fills the enclosing tubular sheath,the core is nonetheless described as a solid core.

Specifically in accordance with the present invention there is provideda detonating cord comprising an elongate tubular sheath encasing a solidcore of an explosive material, the explosive material being comprised ofa first explosive and a diluent. The diluent is present in an amountwhich reduces the velocity of detonation of the detonating cord ascompared to that of an otherwise identical detonating cord in which theexplosive material contains no diluent. In one aspect of the presentinvention, the diluent comprises particles of an explosively inertmaterial, e.g., explosively inert microballoons. The microballoons maybe selected from the class consisting of glass microballoons and resinmicroballoons, preferably the latter, the microballoons having adiameter of from about 10 to about 175 microns. Thus, the microballoonsmay comprise resin microballoons, e.g., phenolic resin microballoons,having a diameter of from about 10 to about 175 microns.

In another aspect of the present invention, diluent comprises a secondexplosive material, e.g., ammonium nitrate, having a lower velocity ofdetonation than the first explosive material.

Another aspect of the present invention provides that the detonatingcord contains from about 0.5 to 15%, e.g., from about 0.5 to 5%, byweight of the diluent, based on the dry weight of the core.

The first explosive may be any suitable explosive, such as one or moreof PETN, HMX, HNS, TNC, PYX and RDX.

A method aspect of the present invention provides an improvement in amethod of cleaving a rock formation. The method comprises drilling aplurality of substantially parallel boreholes into the formation todefine between adjacent boreholes a web of rock interconnecting adjacentboreholes with each other, placing within the boreholes at least onelength of detonating cord extending along the length of the respectiveboreholes, connecting the length of detonating cord to an explosiveinitiating device and initiating the length of detonating cord to cleavethe formation. The improvement comprises that the detonating cord is oneas described above.

Another method aspect of the present invention provides a method formaking a detonating cord as described above. The method comprises thesteps of preparing an explosive material by admixing a first explosivewith a diluent selected from the group consisting of (a) explosivelyinert diluents; (b) a second explosive having a velocity of detonationless than that of the first explosive; and (c) mixtures of (a) and (b),the diluent being present in an amount which reduces the velocity ofdetonation of the detonating cord as compared to an otherwise identicaldetonating cord in which the explosive material contains no diluent. Theexplosive material is enclosed within a tubular sheath to provide adetonating cord having a core of the explosive material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view in elevation of a segment of a slab ofgranite having a plurality of boreholes drilled therethrough;

FIG. 2 is a plan view of the granite slab of FIG. 1 showing an end viewof the boreholes;

FIG. 3 is a schematic view corresponding to that of FIG. 1 showing alength of detonating cord disposed within each of the boreholes andconnected to a trunkline for initiation of the detonating cords;

FIG. 3A is a cross-sectional view, enlarged relative to FIG. 3, of asegment of the length of detonating cord illustrated in FIG. 3;

FIG. 4 is a schematic view corresponding to that of FIG. 1, but showinga stitching arrangement of a continuous length of detonating corddisposed as a loop within the boreholes;

FIG. 5 is a plan view of the granite slab of FIG. 1 after it has beensplit in two by initiation of detonating cord emplaced with theboreholes;

FIG. 6 is a perspective, schematic view of a two-borehole setup forconducting tests of detonating cord; and

FIGS. 7-12 are graphs plotting the pressure output in thousands ofpounds per square inch (“KIPS”) against time in seconds of varioussamples of detonating cord tested in the setup illustrated in FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION AND SPECIFIC EMBODIMENTS THEREOF

As noted above, the velocity of detonation of detonating cord can becontrolled by mixing a diluent with the pulverulent explosive from whichthe explosive core of detonating cord is formed. By utilizing as thediluent inert particulate material or a second explosive less brisantthan the first explosive, i.e., one having a lower velocity ofdetonation than the first explosive, the velocity of detonation of thedetonating cord may be reduced. It has been discovered that by reducingthe velocity of detonation, a reduction in the peak output pressurescaused by detonation of the cord is attained without significantlyaffecting the total energy output of the cord. Such reduction of peakoutput pressure has been found to be highly desirable in certaincircumstances, as it reduces undesired damage to areas of the rockimmediately surrounding the cord. The benefits of the present inventionare clearly shown, for example, in cutting what is referred to as“dimension stone” from in-ground formations of stone such as granite andthe like, or in cutting slabs of dimension stone from larger blocks(sometimes referred to as “production loaves”) of the stone.

This technique of cleaving rock by fracturing the web of intact rockextending between adjacent parallel boreholes is used in a variety ofsituations. In dimension stone quarries, sections of granite or otherdesirable stone are cleaved from the geologic formation. Radialfractures emanating from the boreholes reduce the yield of useablestone. Further, a clean, relatively smooth surface of the rock face fromwhich a segment of rock is cleaved is often desired in applicationsother than dimension stone quarrying. For example, in surface mining andconstruction blasting, a smooth, flat wall of rock in the remainingformation is often desired, e.g., to provide a structurally sound andreasonably smooth wall to minimize rock-fall danger. This smooth-walltechnique is also employed in underground mining and tunnelingapplications to blast a secure, reasonably smooth roof or “back” in themine, the smoothness of which reduces the requirements for mechanicalsupports such as roof bolts and the like.

One aspect of the present invention contemplates the use of anexplosive, e.g., the use of detonating cord, of reduced velocity ofdetonation to cleave rock or stone along a line defined by a series ofboreholes. Detonating cord is waterproof and safer to handle than blackpowder. It typically is a PETN-based explosive and produces none of thehandling problems related to nitroglycerine-based explosives. Detonatingcord can function in very small cord diameters, e.g., as small as lessthan 0.25 inch, i.e., 0.635 centimeter (“cm”), in diameter. Thedisadvantage of conventional, high-velocity detonating cord inapplications designed to form in the cleaved stone, and/or leave behinda smooth, cleaved stone face, is its typically high detonation velocityof up to more than 7000 meters/second, and high radial shock waveoutput. These characteristics, which are conventionally considered to bedesirable attributes necessary to enable conventional detonating cord toinitiate explosive charges of low sensitivity, contribute to excessivefracturing and cracking around the boreholes when conventionaldetonating cord is used to cleave rock. Only the detonating cord of thepresent invention provides an explosive of such small diameter,typically from about 0.125 to 0.250 inch (0.318 to 0.635 cm) diameter,having such a low velocity of detonation.

Another aspect of the present invention utilizes a low-velocitydetonating cord, preferably one having a velocity of detonation lessthan about 5000 meters per second (“m/sec”). The invention enablestaking advantage of the desirable features of detonating cord, such asthe ability to be cut at any point along its length, and its relativelysmall cross-sectional diameter as compared to other explosives, with thefeature of optimizing blasting performance by modifying the velocity ofdetonation of the cord. The low-velocity detonating cord of this aspectof the present invention results in decreased shock loading andincreased gas pressurization within the boreholes in which thedetonating cord is functioned, with no significant reduction in totalenergy output. This characteristic results in reduced radial fracturingaround the periphery of the borehole, more gradual development of thestress field, and more efficient fracturing of the web of rock betweenadjacent, parallel boreholes. In short, the low-velocity detonating cordaspect of the present invention better mimics the action of mechanicalwedges in cleaving stone than do conventional explosives, includingconventional, high-velocity detonating cord, as is demonstrated by thedata provided below. All of these advantages apply to any rock-cleavingapplication. The present invention permits the radial output energy ofdetonating cord to be tailored to a specific blasting application bychanging, e.g., reducing, the velocity of detonation of the cord.

Referring now to FIG. 1, there is shown a somewhat schematiccross-sectional view taken along line I-I of FIG. 2 of a granite block10 having a plurality of boreholes 12 formed therein and extendingsubstantially parallel to each other from and through top surface 14 ato and through bottom surface 14 b of granite block 10. As best seen inFIG. 2, parallel boreholes 12 are aligned along a straight line todefine a web 16 of stone extending between and connecting boreholes 12to each other. Web 16 is indicated in FIG. 2 by dash lines.

Generally, it is desirable to minimize the number of boreholes needed toeffectuate a particular task in order to reduce costs of drilling.Reducing the number of boreholes, and thereby increasing the spacingbetween boreholes for a given length of rock formation to be cleaved,increases the amount of explosive required per borehole. The increasedamount of explosive per borehole is necessary in order to ensurebreakage of the increased length of the web of rock between adjacentboreholes, but also increases radial fractures and collateral damage ofthe cleaved rock. Conversely, reducing the explosive load of eachborehole requires decreasing the spacing between adjacent boreholes, andtherefore increasing the number of boreholes and consequently increasingthe drilling costs. The present invention, by providing an explosivewhich generates reduced peak shock wave pressures without a substantialreduction in total energy output, enhances the ability of the explosiveof the present invention to break the rock web while reducing collateralradial damage to the rock. This enables increasing the spacing betweenadjacent boreholes without a corresponding increase in collateraldamage.

FIG. 3 shows schematically one arrangement for providing throughout thelength of each of boreholes 12, lengths 18 of reduced-velocitydetonating cord in accordance with an embodiment of the presentinvention. In the embodiment schematically illustrated in FIG. 3,individual lengths 18 of reduced-velocity detonating cord are connectedto a trunkline 20 which may itself comprise detonating cord and whichmay, but need not, be reduced-velocity detonating cord in accordancewith an embodiment of the present invention. Trunkline 20 is initiatedat one end thereof by any suitable known means (not illustrated) and adetonation signal travels along trunkline 20 to initiate each of thelengths 18 of detonating cord to fracture the web 16 of stone.

FIG. 3A shows that detonating cord 18 comprises a tubular sheath 18 aencasing a solid core 18 b of explosive material throughout which isdispersed a particulate diluent, provided in the illustrated embodimentby microballoons 18 c. Each of microballoons 18 c is a hollowparticulate body enclosing a void containing a gas, e.g., air. Tubularsheath 18 a is made of any suitable material to provide adequatemechanical strength and to be resistant to penetration of water or otherliquids into core 18 b.

FIG. 4 shows an alternate arrangement, sometimes referred to as“stitching”, in which a single length 22 of reduced-velocity detonatingcord in accordance with an embodiment of the present invention isthreaded in a serpentine, stitching-like arrangement through each ofboreholes 12 by inserting a return-loop of detonating cord throughsubstantially the entire length of each borehole. Length 22 ofdetonating cord is initiated by any suitable means (not illustrated) anddetonates along the length thereof to fracture the web 16 of stone. Inthis arrangement the total length of reduced-velocity detonating cord ina borehole is effectively doubled as compared to the arrangement of FIG.3. Therefore, other factors being equal, the diameter of thereduced-velocity detonating cord used in the arrangement of FIG. 4 wouldbe about one-half that of the reduced-velocity detonating cord used inthe arrangement of FIG. 3.

The result of fracturing web 16 is schematically illustrated in FIG. 5,which is a plan view corresponding to that of FIG. 2, but showing thegranite block after it is cleaved in two by either the lengths 18 ofdetonating cord as illustrated in FIG. 3, or the length 22 of detonatingcord as illustrated in FIG. 4. Granite block 10 (FIG. 2) has now beensplit into two granite blocks, 10 a and 10 b (FIG. 5). Adjacent the arcs12 a, 12 b (FIG. 5) of the former boreholes 12 (FIG. 2) are a series ofsmall radial cracks 24 and a few considerably longer radial cracks,referred to as “stickers” in the dimension stone industry, shown at 26.In order to reduce the amount of wasted stone, it is, of course, desiredthat the radial cracks 24 and stickers 26 be reduced in number and/orshortened as much as possible.

A series of blocks was cleaved from larger blocks (production loaves) ofgranite using either 18 gr/ft (3.8 g/m), low-velocity detonating cord inaccordance with an embodiment of the present invention, or conventional18 gr/ft (3.8 g/m) detonating cord. All holes in the test blocks wereloaded with Viking B-gel, an inert gel product with microballoons,available from Viking Explosives & Supply, Inc. of Rosemont, Minn. TheB-gel is a product which is well known for use as a coupling agent forthe purpose of buffering the initial shock pressure generated byfunctioning of the detonating cord. This or an equivalent gel product isused in an effort to reduce unwanted radial fracturing. A total of fourblocks were cleaved using reduced-velocity detonating cord in accordancewith an aspect of the present invention, and three blocks were cleavedusing conventional high-velocity detonating cord. All blocks were sawedinto 6 to 8 inch (about 15.2 to 20.3 cm) thick slabs and then polished.The yield from each slab, along with the degree of radial fracturing,was determined through observation and digital imaging analysis in orderto compare the performance of each type of detonating cord. The observedresults of these tests are tabulated in the following Table A. Theaverage length of stickers was twelve inches.

TABLE A Average Number Average Average of Holes Minimum Maximum AverageAffected by Crack Crack Number Cracking, Length Length of Stickers perSlab (Inches) (Inches) per Slab A. Reduced- 17.0 2.4 4.1 0.5 VelocityDetonating Cord B. Comparative 18.3 3.6 5.7 2.9 High-Velocity DetonatingCord Difference  1.3 1.2 1.6 2.4 % Improvement 7% 32% 28% 82% ProvidedBy A Over B

It is seen from Table A that the reduced-velocity detonating cord inaccordance with an embodiment of the invention, Sample A, having avelocity of detonation of 4,800 m/sec, significantly reduced cracking ascompared to comparative high-velocity detonating cord, Sample B, havinga velocity of detonation of 7,100 m/sec. The detonating cord of Sample Ahad a loading of 18 gr/ft (3.83 g/m) of 90% by weight PETN, 5% by weightammonium nitrate, and 5% by weight of phenolic microballoons having aparticle size distribution shown in Table B, below. (The percents byweight are on the basis of percent by weight of the combined weight ofPETN, ammonium nitrate and phenolic microballoons.) The comparativedetonating cord of Sample B had a loading of 18 gr/ft (3.83 g/m) of 100%PETN, i.e., it contained no diluent.

Blast-pressure profiles were measured for a series ofcord/coupling-agent combinations, The test arrangement is schematicallyillustrated in FIG. 6 in which two test boreholes, 28 and 30,terminating in respective borehole bottoms 28 a and 30 a, were bored inthe stone parallel to each other with their peripheries spaced 6 inches(15.24 cm) from each other, this distance being illustrated as D in FIG.6. A test detonating cord 32 was inserted throughout the length of testborehole 28 and, in those cases in which a coupling agent other than airwas utilized, borehole 28 was filled with a coupling agent, e.g., gel orwater (not shown). A loop (not shown) of test detonating cord 32 wasinserted into borehole 28 in order to test a “stitching” arrangement ofdetonating cord, as schematically illustrated in FIG. 4. Test borehole30 was filled with water as a coupling agent, and an underwater blastpressure sensor 34 was placed within the water-filled test borehole 30.Underwater blast pressure sensor 34 was connected by a cable 36 tocomputerized recording equipment (not illustrated) to record “pressureprofiles”, i.e., graphs of the pressure of the shock wave or pressurepulse generated by detonation of test detonating cord 32 as a functionof time. The pressure profiles generated from test detonating cord 32are shown in FIGS. 7-16, wherein the shock wave pressures are plotted onthe left-hand vertical axes in kilopounds per square inch (“KIPS”) andon the righthand vertical axes in kilograms per square centimeter(“kg/cm²”). The time after functioning (detonation) is plotted on thehorizontal axes in seconds. Among the tested detonating cords 32 were 18gr/ft (3.83 g/m) low-velocity cord in accordance with an embodiment ofthe present invention, and 7.5 gr/ft (1.60 g/m) and 18 gr/ft (3.83 g/m)comparative high-velocity cords. In some tests, in lieu of water as thecoupling agent media in test borehole 28, B-gel or air were used as thecoupling agent.

The tests resulted in fracturing of the stone adjacent the boreholes inthe manner described above in connection with FIG. 5. Thus, radialfractures (such as those shown at 24 in FIG. 5), which tended to be ofrelatively constant length, affected many of the boreholes on theperimeter of the slab. Stickers (such as those shown at 26 in FIG. 5)tended to be fewer in number, but up to 2 to 10 times the length of, theradial cracks 24. The slabs shot with reduced-velocity detonating cordwere observed to have 30% shorter radial fractures and 80% fewerstickers than slabs shot with comparative, conventional high-velocitydetonating cord. Both of these cord types were used with B-gel as thecoupling media.

The results of the pressure testing shown in FIGS. 7-12 support thevisual observations of the cleaved stone. In the pressure measurementsillustrated in FIGS. 7-12, the shock pulse from the detonating cord isseen as a very sharp and high peak close to the front (earliest timeafter detonation) of the pressure profile. This pressure pulserepresented by this peak is believed to be the primary cause of radialfracturing around the boreholes, and the magnitude of this earlypressure pulse is believed to be proportional to the length and quantityof the resultant radial fractures and stickers. The key to reducingradial fracturing while breaking the web of rock connecting theboreholes is to reduce the high-amplitude, sharp peaks while maintainingmuch of the pressure that occurs generally throughout the pressureprofile. Maintaining the overall level of the pressure profile avoidsreducing the total energy output or work while eliminating or reducingshock wave pressure peaks. The energy output or work is indicated by thearea beneath the curve in the Figures plotting the pressure profiles.These areas represent the product of pressure multiplied by time.

A pressure profile for 18 gr/ft (3.83 g/m) comparative high-velocitydetonating cord in water is shown in FIG. 7. (In FIGS. 7-12, repeatedtests are shown on the same set of axes and it is seen that the resultsare reproducible with only very minor variations.)

The pressure profile of FIG. 8 shows the pressure profile fromcomparative high-velocity detonating cord in B-gel coupling agent, andthat of FIG. 9 shows the pressure profile from low-velocity detonatingcord in accordance with an embodiment of the present invention in B-gelcoupling agent. A comparison of FIGS. 8 and 9 makes clear that thelow-velocity detonating cord did not produce the sharp, high-amplitudepressure transients that were typical of the comparative high-velocitydetonating cord, while otherwise maintaining a comparable level ofpressure.

The pressure profile of the low-velocity detonating cord in water shownin FIG. 10 also shows that the reduction of the sharp peaks was notaccompanied by a general reduction of pressure throughout the trace ofthe pressure profile. The performance of the low-velocity detonatingcord in water shown in FIG. 10 is of particular interest. It will benoted that these pressure profiles are very similar to those measuredwith low-velocity detonating cord in B-gel (shown in FIG. 9) andrepresent a dramatic improvement over comparative high-velocitydetonating cord in B-gel, shown in FIG. 8. These results indicate thatlow-velocity cord in water performs better than the comparativehigh-velocity detonating cord in B-gel, and about as well aslow-velocity detonating cord in B-gel. Given the high cost of B-gelcoupling agent and the difficulty of mixing and using the product inday-to-day operations, the low-velocity detonating cord in accordancewith an embodiment of the present invention provides greatly enhancedperformance and may enable elimination of the need to use B-gel.

FIG. 1 IA shows the pressure profile for a comparative high-velocity 18gr/ft (3.83 g/m) detonating cord in air and a low-velocity 18 gr/ft(3.83 g/m) detonating cord in accordance with an embodiment of thepresent invention in air, the cords having been placed in the testborehole (28 in FIG. 6) using the stitching arrangement schematicallyillustrated in FIG. 4. Using the stitching arrangement of FIG. 4provided a doubled length of the test detonating cord within the testborehole. In addition, a thimble- or cup-shaped spacer (not shown in thedrawings) was emplaced at the bottom of the test borehole atop thereturn-bend portion of the looped detonating cord in order to ensurethat the detonating cord extended the full length of the borehole. Therespective legs of the doubled detonating cord emerged from under thespacer at diametrically opposite sides of the periphery thereof,resulting in each leg of the detonating cord being held against, or atleast in close proximity to, the wall of the borehole, thereby enhancingcoupling of the detonating cord to the walls of the borehole. As seen bycomparing FIGS. 11A and 11B to each other, the contrast between the twotypes of cords is striking. The high peak loads of pressure engenderedby the comparative high-velocity detonating cord shown in FIG. 11A areabsent from the pressure profile of the low-velocity detonating cord inaccordance with an embodiment of the present invention shown in FIG.11B. A comparison of the pressure profiles of FIGS. 11A and 11B alsoshows that the low-velocity detonating cord (FIG. 1 IA) nevertheless hadabout the same general level of pressure output as the comparativehigh-velocity detonating cord (FIG. 11B). The significant difference isthe very desirable elimination of the high-pressure peaks shown in FIG.11B.

FIG. 12 shows the pressure profile of a comparative 7.5 gr/ft (1.60 g/m)high-velocity detonating cord in water as a coupling agent. The initialpressure peak, while reduced as compared to higher core load detonatingcords, is nonetheless prominent, about three times higher than theremaining non-peak pressure level.

While any suitable diluent may be utilized to reduce the velocity of thedetonating cord, one which is found to be particularly useful isphenolic microballoons, e.g., of the type conventionally used as afiller in fiberglass resin applications to lower weight and density offinished fiberglass items. Glass microballoons are commonly used inblasting agents, but glass functions as a sensitizing agent in dryexplosives and therefore is dangerous and much less desirable thanphenolic microballoons for utilization in a detonating cord. Phenolicresin is a brittle material and the addition of phenolic microballoonsto the explosive core does not appear to sensitize dry PETN. Phenolicresin has a specific gravity of approximately 1.35 and the phenolicmicroballoons used had a tapped bulk density of 0.13 grams per cubiccentimeter. (The container of phenolic microballoons is tapped on asolid surface to settle the material prior to measuring its density. Thedensity of the settled material is referred to as its “tapped bulkdensity”.)

As indicated above, the diluent may comprise an explosive diluent, suchas ammonium nitrate, which is of significantly lesser brisance than themajor explosive ingredient, e.g., PETN, of the detonating cord or otherexplosive used in the practices of the present invention. The explosivediluent may be used in combination with another diluent such as phenolicor other microballoons, or the explosive diluent or other type or typesof non-explosive diluent, e.g., microballoons, may be used as the solediluent. The use of a less brisant explosive such as ammonium nitrate asthe sole diluent in a detonating cord has the disadvantage that theamount of less brisant explosive required to attain a sufficientreduction in velocity of detonation will significantly increase thediameter of the detonating cord.

The phenolic microballoons used to prepare the tested low-velocitydetonating cord had an average particle size distribution as follows,wherein μ stands for microns.

TABLE B % of Maximum Particles Diameter 75% 71μ 50% 51μ 25% 35μ

It was determined by experimentation that the step of compressingdetonating cords in the course of post-manufacture inspection forvariations in cord diameter resulted in rupturing some of the phenolicmicroballoons used as a diluent in the low-velocity detonating cord ofthe present invention. Consequently, these compressed cords did notexhibit the reduction in velocity of detonation (“VOD”) expected fromthe addition of the quantity of phenolic micro balloons. This is shownby a comparison of the data for the second and third entries in Table 1wherein a significantly smaller reduction in VOD is shown for compressedcord containing the same amount of microballoons than for uncompressedcord. In Table 1, “SFPETN” means a superfine grade of PETN, having aparticle size of about 20 microns diameter.

TABLE 1 Core Load % By Weight Inspection- VOD gr/ft g/m MicroballoonCompressed PETN Grade (average) 18 3.83 0% Yes SFPETN 6513 m/s 18 3.835% Yes SFPETN 5515 m/s 18 3.83 5% No SFPETN 4795 m/s

It was determined by experimentation that simply diluting the PETN in200 gr/ft (42.6 g/m) detonating cord with ammonium nitrate reduced theVOD slightly. However, the addition of microballoons without compressionof the detonating cord for examination purposes substantially reducedthe VOD as shown in Table 2. Reference to “MFPETN” in Table 2 means amedium fine grade of PETN, having a particle size of about 150 micronsdiameter.

TABLE 2 Core Load % By Weight gr/ft g/m AN* Microballoon PETN Grade VODAverage 200 42.6 0% 0% MFPETN 6758 m/s 200 42.6 15% 0% MFPETN 6247 m/s200 42.6 0% 10% MFPETN 4034 m/s *AN = ammonium nitrate

Generally, any suitable quantity of diluent may be used to attain adesired change, i.e., a reduction, in velocity of detonation. Forexample, phenolic microballoons (or other diluents) may be added inamounts of from about 0.5 to 5%, 0.5 to 10%, 0.5 to 15%, 1 to 5%, 1 to7%, 1 to 10% or 1 to 15% by weight of the combined weight of explosiveand diluent.

In the experimental results shown in Tables 1 and 2, adding to the PETNpowder an ammonium nitrate diluent (Table 2) reduced the VOD onlyslightly. On the other hand, adding phenolic microballoons as thediluent (Tables 1 and 2) substantially reduced the core density of thedetonating cord and the VOD. This is attributed to the fact that themicroballoons reduced the density of the PETN much more than did theammonium nitrate which had little or no effect on density.

When the same weight percent of phenolic microballoons was added to theexamined (post-manufacture compressed) and unexamined (notpost-manufacture compressed) detonating cords, a loss of some of the VODreduction effect was noted in the examined (compressed) samples. Thisloss of desired reduction in VOD is attributed to crushing of themicroballoons by the post-manufacture compression for inspectionpurposes. Since compressing the cord increased the VOD without, ofcourse, changing its content by weight of phenolic material (crushed andnon-crushed phenolic microballoons), the shape of the microballoons,i.e., their effective density, must be involved in the loss of VODreduction. It was verified, but not quantified, that a substantialpercentage of the microballoons were ruptured and compacted by thepost-manufacture compression examination, thus increasing the density ofthe phenolic material in the compressed samples as compared to that inthe uncompressed samples. A more substantial VOD reduction effect isbelieved to be achieved by reduction of density of the explosive core ofthe detonating cord, not merely dilution of the explosive powder with aninert material or a less brisant explosive which has no or very littledensity-reducing effect.

While the invention has been described in detail in connection withspecific embodiments thereof, it will be appreciated that neither theinvention nor the appended claims are limited to the specificillustrative embodiments.

1. A detonating cord comprising an elongate tubular sheath encasing acore of an explosive material comprising a first pulverulent explosiveadmixed with a diluent comprising explosively inert microballoonspresent in an amount which reduces the velocity of detonation of thedetonating cord as compared to that of an otherwise identical detonatingcord in which the explosive material contains no explosively inertmicroballoons.
 2. The detonating cord of claim 1 wherein themicroballoons are selected from the class consisting of glassmicroballoons and resin microballoons, the microballoons having adiameter of from about 10 to about 175 microns.
 3. The detonating cordof claim 1 wherein the microballoons comprise resin microballoons. 4.The detonating cord of claim 3 wherein the microballoons have a diameterof from about 10 to about 175 microns.
 5. The detonating cord of claim 4wherein the microballoons comprise phenolic resin microballoons.
 6. Thedetonating cord of claim 1 wherein the explosive material furthercomprises a second explosive having a lower velocity of detonation thanthe first explosive.
 7. The detonating cord of claim 6 wherein thesecond explosive comprises ammonium nitrate.
 8. The detonating cord ofclaim 1 containing from about 0.5 to 15% by weight of the diluent, basedon the dry weight of the core.
 9. The detonating cord of claim 8containing from about 0.5 to 5% by weight diluent.
 10. The detonatingcord of claim 1 wherein the first explosive is selected from the classconsisting of PETN, HMX, LINS, TNC, PYX and RDX, and mixtures of two ormore thereof.
 11. A method of cleaving a rock formation comprising:drilling a plurality of substantially parallel boreholes into theformation to define between adjacent boreholes a web of rockinterconnecting adjacent boreholes with each other; placing within theboreholes at least one length of detonating cord extending along thelength of the respective boreholes; connecting the length of detonatingcord to an explosive initiating device and initiating the length ofdetonating cord to cleave the formation; wherein the detonating cordcomprises an elongate tubular sheath encasing a core of an explosivematerial comprising a first pulverulent explosive admixed with a diluentcomprising explosively inert microballoons present in an amount whichreduces the velocity of detonation of the detonating cord as compared tothat of an otherwise identical detonating cord in which the explosivematerial contains no explosively inert microballoons.
 12. The method ofclaim 11 wherein the explosive material contains from about 0.5 to 15%by weight of the diluent, based on the dry weight of the core.
 13. Themethod of claim 11 wherein the explosive material contains from about0.5 to 5% by weight of the diluent, based on the dry weight of the core.14. The method of claim 11 wherein the first explosive is selected fromthe group consisting of PETN, HMX, HNS, TNC, PYX and RDX.
 15. The methodof claim 11 wherein the diluent further comprises a second explosivematerial having a lower velocity of detonation than the first explosivematerial.
 16. The method of claim 11 wherein the microballoons compriseresin microballoons having a diameter of from about 10 to about 175microns.
 17. A method for making a detonating cord comprises the stepsof preparing an explosive material by admixing a pulverulent explosivewith a diluent comprising explosively inert microballoons, the diluentbeing present in an amount which reduces the velocity of detonation ofthe detonating cord as compared to an otherwise identical detonatingcord in which the explosive material contains no explosively inertmicroballoons; and enclosing the explosive material within a tubularsheath to provide a detonating cord having a core of the explosivematerial.
 18. The method of claim 17 including admixing a sufficientquantity of the explosively inert microballoons with the first explosiveto provide in the core from about 0.5 to 15% by weight of theexplosively inert microballoons, based on the dry weight of the core.19. The method of claim 17 or claim 18 wherein the first explosive isselected from the class consisting of PETN, HMX, HNS, TNC, PYX and RDX,and mixtures of two or more thereof.
 20. The method of claim 17 or claim18 wherein the microballoons comprise resin microballoons.
 21. Themethod of claim 20 wherein the microballoons comprise phenolic resinmicrobal loons.
 22. The method of claim 17 or claim 18 wherein themicroballoons have a diameter of from about 10 to about 175 microns.